(a) Field of the Invention
The present invention relates to an equipment for air distillation, of the type comprising an air distillation double column which comprises a mean pressure column and a low pressure column, and a column for the production of impure argon connected to the low pressure column and containing a main head condenser cooled by vaporizing expanded rich liquid extracted in the vat portion of the mean pressure column.
(b) Description of Prior Art
The known solution for producing argon consists in withdrawing, through a so-called argon tapping duct located at an intermediate level of the low pressure column, a vapor in which the argon concentration is in the vicinity of 10% and having a low nitrogen concentration (&lt;0.1%). This vapor is sent to the vat portion of a column for the production of impure argon, so-called "mixture column", and is concentrated into its lighter components (N.sub.2 + Ar) by contact through plates with a liquid which becomes loaded with oxygen. This liquid is obtained by liquefying a portion of the column head vapor, in a condenser in which the refrigeration is supplied by vaporizing, under low pressure, the rich liquid withdrawn in the vat portion of the mean pressure column, after sub-cooling at about -185.degree. C.
The argon mixture (impure argon) withdrawn in the head portion of the mixture column has a flow about 30 times smaller than the argon tapping flow and has the following typical composition (in moles):
______________________________________ N.sub.2 3% Ar 95% O.sub.2 2% ______________________________________
The vat liquid of the mixture column is sent back to the low pressure column.
The final step for producing pure argon in liquid form consists, in a first stage, to eliminate oxygen in the form of water by catalytic conversion, into a so-called "DEOXO" system, in the presence of excess hydrogen according to the reaction: EQU H.sub.2 + O.sub.2 .fwdarw.H.sub.2 O
After drying, the mixture, which now contains only nitrogen, argon and traces of hydrogen, is cooled down and sent to a distillation column where argon in liquid form is found in the vat and the lighter gas, in the head.
To provide for a good operation of the "DEOXO" system, and also to limit the consumption of hydrogen in this apparatus, the molar concentration of hydrogen in the mixture preferably does not exceed 2 to 3 percent. Moreover, to ensure the reflux of the mixture column, in the condenser, there should be a certain difference of temperature between the rich liquid which is vaporized and the vapor in the column head. Therefore, this head vapor should not be too rich in nitrogen, and consequently the content of tapping nitrogen, which is about 30 times lower because of the very high reflux rate, should be very small. A nitrogen content of the order of 0.1% in tapping argon, which gives 0.1%.times.30=3% in the argon mixture, is acceptable.
If this condition presents no problem in the designing of new equipment, this is not the case when means for producing argon is to be added to an existing double column in which there is no provision for such addition.